Sometimes it's a secret room. Sometimes it's a whole playable level. Sometimes it's a secret message.
The first Easter egg appeared in a video game called "Adventure," which was released in 1979 for the Atari 2600 console. The game's author was annoyed he didn't get credit for his work, so he buried the text "Created by Warren Robinett" in a room so deep in the game, it wasn't found until a year later.
Since then, there have been hundreds of Easter eggs hidden in video games. And we've scoured the web to bring some of the coolest ones to light. (We've tried not to include any spoilers, but if you haven't played some of the games mentioned, read at your own risk.)